Protein Kinase C-alpha Overexpression Stimulates Akt Activity and Suppresses Apoptosis Induced by Interleukin 3 Withdrawal

To investigate the role of protein kinase C (PKC) in apoptotic signaling induced by cytokine withdrawal, we expressed PKC-alpha, -delta and -epsilon individually in the 32D myeloid progenitor cells. The parental and PKC-delta- and PKC-epsilon-transfected 32D cells underwent apoptosis within 24 h in the absence of interleukin 3. In contrast, expression of PKC-alpha inhibited the onset of apoptosis as determined by genomic DNA fragmentation and flow cytometric analysis. Correlating with the inhibition of apoptosis, PKC-alpha transfectants exhibited increased activity of the endogenous Akt serine/threonine kinase. Furthermore, PKC-alpha, but not PKC-delta or -epsilon, specifically activated overexpressed Akt. PKC-alpha-induced Akt activity was partially dependent on phosphoinositol 3' kinase (PI 3'K) since a PI 3'K inhibitor was able to suppress PKC-alpha-induced Akt activation. Both basal and interleukin 3-stimulated phosphorylation of Akt on serine 473 was enhanced in the PKC-alpha and Akt contransfectants. Coexpression of wild type Akt and PKC-alpha resulted in greater suppression of apoptosis than PKC-alpha expression alone. Together, our results demonstrate that suppression of apoptosis by PKC-alpha correlates with its ability of activating endogenous Akt. Furthermore, activation of overexpressed Akt by PKC-alpha is consistent with their synergistic effect on suppressing apoptosis, providing the strong evidence of cross talk between Akt and PKC-alpha.